Position of f-stop

[multiweb (Marc)]

When you place a mask in an optical train to reduce the aperture does it matter how far from the refracting/reflecting surface you place it? I mean in the example of lenses why is the aperture control within the glass of the lens? Would stopping the lens at the back or at the front make any difference?

[DavidU (Dave)]

http://www.cloudynights.com/ubbthrea.../o/all/fpart/1

[multiweb (Marc)]

Thanks for the link David. Looks like stopping before the lens works quite well. I have a couple of step down rings to do that. I have to admit I don't either really like those 6,8 or more spikes on stars when stopping the lenses.

[Wavytone]

The aperture stop should be located at the geometric entrance pupil of a lens in order to minimise the effect of vignetting across the field of view. This point is also known as the lens vertex. For taking a panorama, the lens should be rotated around this point in order to have no parallax mis-matches across adjoining frames.

If you put an aperture stop in front of a lens which may already has an internal aperture stop, there are several results:

a) you will introduce vignetting (ie dark corners). To minimise this the aperture should be as close as possible to the lens.

b) it will alter the appearance of off-axis images of point sources (ie stars) if the lens has small amounts of spherical aberration or coma.